What are F-Stops?

(Mitch Alsup) writes:

Bob wrote in message . ..
On Fri, 27 Aug 2004 02:50:28 GMT, "greg" wrote:

The best lens we could have would be f 1. I hear NASA actually made one... any
less then 1 is impossible since it would be inside out...

Canon I-Line lithographic steppers use 22 individual Flourite lenses
to make a 5X reduction print 'lens' with an F/ratio of F/0.4 that is
diffraction limited across a 18mm field at 265nm (deep UV). This is
what the silicon guys use to print 0.25, 0.18, 0.13, currently 0.090
(90 nm) and soon 0.065 (65 nm) chips. This lens has a depth of field
only a few dozen nano meters deep.

I rather suspect that you, or someone else, has confused F-number with
NA in this example.

B

On Sat, 28 Aug 2004 06:07:06 GMT, Matt Ion wrote:

Bob wrote:

The best lens we could have would be f 1. I hear NASA actually made one... any
less then 1 is impossible since it would be inside out...

If I remember rightly, one of the big selling points of the Canon EOS
line when it first came out was that the lens mount was significantly
larger than anything previously available on a 35mm SLR - large enough
to accomodate the first f/1 50mm lens.

BTW, lower f-stops are certainly possible: a 50mm f/.8 lens would simply
be a lens with a 50mm focal length and a 62.5mm aperture (50/.8).

I know - I was mistakenly thinking about the impossibility of putting one into a
camera - not the impossibility of making one!

BTW the last number on the lens doesn't count... a lens of 3.5 just
happened to
be that size, and that's the biggest it can be! It is not a full f stop away
from the next standard number. (f4)

That didn't make any sense at all...

Sure it does... as you yourself not, the series of F-stops goes 11, 5.6, 4, and
then 3.5... it should be 2.8.... but the lens is a 3.5...

see?


Commonly "marked" f-stops generally start at f/1 and go up by full stops
(ie. 1/2 reductions in light). As you state, a full stop means a ratio
of 1.4 in the diameter of the aperture, so we get full stops at f/1,
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44, etc
(every doubling or halving in aperture is two stops). The number listed
on a lens is the maximum opening that lens is capable of, and it doesn't
have to be on an exact stop (a 100mm, f/3.5 lens would have a max
aperture of about 28.57mm).

doesn't work that way... the standard series is always used...

Bob wrote:

The best lens we could have would be f 1. I hear NASA actually made one...
any
less then 1 is impossible since it would be inside out...

Guess again! Canon made a normal focal length lens with max aperture of f/0.95
back around the early 70's. The f/stop is simply the ratio of the focal length
to the aperture's diameter.

Bob wrote:

The best lens we could have would be f 1. I hear NASA actually made one...
any
less then 1 is impossible since it would be inside out...

Guess again! Canon made a normal focal length lens with max aperture of f/0.95
back around the early 70's. The f/stop is simply the ratio of the focal length
to the aperture's diameter.

In article , Wilt W
writes
Bob wrote:

The best lens we could have would be f 1. I hear NASA actually made one...
any
less then 1 is impossible since it would be inside out...

Guess again! Canon made a normal focal length lens with max aperture of f/0.95
back around the early 70's. The f/stop is simply the ratio of the focal length
to the aperture's diameter.

The theoretical maximum aperture of any lens in air, when focussed on a
subject at infinity*, is f/0.5 (NA** = 1.0). In practice this is
unattainable, f/0.7 is the practical limit, and only a few lenses***
have ever exceeded this limit (and not by much). However, lenses of
f/0.75 or so are not uncommon in specialist applications.

For a lens immersed in a medium having the same refractive index as the
glass - as is often done for high-powered microscope lenses - the
theoretical limit depends on the refractive index, but the practical
limit is f/0.36 (NA = 1.4). I have two lenses of this aperture.

* It is often overlooked that the f-number is dependent on the
object/image conjugate distances; as you move the lens away from the
film/sensor to focus on a subject closer than infinity, the aperture (as
seen by the sensor, which is what matters) becomes smaller.

**NA, or numerical aperture, is the designation used for the aperture
microscope lenses and in certain other areas; in essence, f-number =
1/(2xNA).

***Outside the area of microscopy, where dry (i.e. non-immersion) lenses
of NA 0.95 (f/0.53) are common. They are however a pain to use
correctly, requiring cover glasses to be within 0.01mm of the design
thickness.

David
--
David Littlewood

British condoms.

-- Larry

ROTFLOL!

Larry wrote:
British condoms.

-- Larry